1. Field of the Invention
The present invention relates to an image signal processing apparatus for processing an image signal, for example, an image printing apparatus, an image transmitting apparatus or an image storing apparatus.
2. Related Art
Video printers which are able to print a video frame image on a printing paper are well known. These video printers' printing areas are different in accordance with a kind of input device which generates a TV signal as shown in FIG. 4. In other words, valid image signal periods, i.e., from start timing positions ST to end timing positions ED as shown in FIG. 4, are different both in the exact position of the valid image information within the horizontal scan line signal, and, perhaps to a lesser degree, in the fraction of the duration of the horizontal scan line signal made up by the valid image information, in accordance with the kind of the input device.
In FIG. 4, a signal 100 is a video signal from a tri-tube video camera, and a signal 101 is a video signal from a camera for a video recording apparatus. The signals 100 and 101 are different as to location of the valid image period within the period between the successive horizontal synchronizing signals H sync. So, if printing is started immediately after a horizontal blanking period 111 of the signal 101, that is, from a rising 110 of H sync, black will be printed between the end point of the blanking period and the starting point of the valid image period, such as in picture 114. On the other hand, if printing is continued to a starting point of front porch period 113 of the signal 100, a black line will be formed at the right side of a print frame as in picture 115. These black lines spoil the resulting print picture.
In order to prevent formation of the black line, the print starting point and print end point can be shifted by several percent (5 to 10%). However, if this is done, an edge portion of the important valid image will be lost, and will not be reproduced, as shown in print picture 116 in FIG. 4. In other words, in the conventional video printer more than 10% of the actual image information will be lost, and the edge portion of the valid image will not be printed. It is a severe problem that the entire image information cannot be reproduced, for example, in a case of using image information in publishing a newspaper.
In the case that the image signals from the input devices are transmitted as still video signals, the image signals which are generated immediately after the horizontal blanking period 111 of the signal 101 has passed from the rising 110 of the H sync, and until the starting point of the front porch period 113 of the signal 10, are written into a frame memory and all the data stored in the frame memory is transmitted, conventionally.
Thus, some amount of invalid image data and blank image data of the pedestal level is also sent with valid image data, so that the efficiency of transmission is extremely poor.
In order to avoid such waste, the transmission starting point and transmission end point can be shifted by several percent (5 to 10%). If this is done, however, an edge portion of the important valid image will be lost, as in printing, so that this is not a good solution.
Such a problem occurs not only in a video printer and in a transmission apparatus, but also in writing image information into an image memory.